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doc: add documentation on printing kernel addresses

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Hashing addresses printed with printk specifier %p was implemented
recently. During development a number of issues were raised regarding
leaking kernel addresses to userspace. Other documentation was updated but
security/self-protection missed out.

Add self-protection documentation regarding printing kernel addresses.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Tobin C. Harding 2017-12-20 08:17:17 +11:00 committed by Jonathan Corbet
parent da271403a8
commit 227d1a61ed
1 changed files with 15 additions and 0 deletions
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15
Documentation/security/self-protection.rst
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@ -270,6 +270,21 @@ attacks, it is important to defend against exposure of both kernel memory
addresses and kernel memory contents (since they may contain kernel
addresses or other sensitive things like canary values).
Kernel addresses
----------------
Printing kernel addresses to userspace leaks sensitive information about
the kernel memory layout. Care should be exercised when using any printk
specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb]
in certain circumstances [*]). Any file written to using one of these
specifiers should be readable only by privileged processes.
Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1
addresses printed with the specifier %p are hashed before printing.
[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is
printed. If KALLSYMS is not enabled the raw address is printed.
Unique identifiers
------------------